In a batch melting furnace, efficiency is increased when the heat from the furnace is passed in heat-exchange relationship with the batch material being supplied to the melting furnace. The batch can thus be preheated to elevated temperatures to save significant amounts of energy subsequently required to melt the batch.
Preferably, the heat-softenable batch material is in the form of balls or pellets in the heat-exchange chamber through which the hot gases are passed. However, it has been discovered that the pellet size must be substantially uniform. Otherwise, pellets of varying sizes tend to nest and provide excessive restriction to the flow of the gases past the pellets in the chamber. It has also been discovered that pellet size is important in addition to uniformity. If the pellets are too small, again undue restriction to the flow of the hot gases results. If the pellets are too large, their surface-to-weight ratio is accordingly reduced and the heat transferred to them is accordingly decreased. Also, trapped moisture in the larger pellets may turn to steam and cause the pellets to explode. Specifically, it has been found that pellets of one-half inch nominal diameter with a range from three-eights inch to five-eights inch in diameter are the ultimate for obtaining maximum heat transfer from the hot exhaust gases to the pellets.
The pellets of the heat-softenable batch material preferably are made in a modified commecially-available pelletizer. The components of the batch are mixed together and then supplied to the pelletizer. During transportation to the pelletizer, the batch components tend to segregate so that the actual batch supplied to the pelletizer will vary, even though the final pellets produced and supplied to the melting furnace or unit average out so that the short variations are not material. However, the short variations in the batch components tend to affect the pellet-forming ability of the batch and the size of the pellets produced, other factors being constant. The feed rate of the batch to the pelletizer will also vary and thereby also affect pellet forming and pellet size. Liquid, and specifically water, is also supplied to the pelletizer near the batch supply. With the batch component or quantity variation, different size pellets will result when the water quantity is held constant. However, it has been found that the water quantity, or the ratio of the batch to the water, will also affect the pellet size, with more water resulting in larger pellets and less water resulting in smaller pellets, at least in most instances.
In the prior art, considerable difficulty has been experienced with pelletizing. An acceptable manner of controlling the size of the pellets produced was experienced.
The difficulty has resulted in an uneconomical operation of a pelletizer where an excessively large portion of the pelletizer output falls above or below acceptable size limits and must be reground for recycling through the pelletizer.